Before the acrobats of Parkour and the gonzo activists of free-running, before the situationist-anarchists of skateboarding in California's Dogtown, who covertly drained suburban swimming-pools to ride their sky-blue curves, there was Philippe Petit.

This was the 24-year-old French highwire artiste who loved to trespass on famous high buildings and ply his marvellous trade, stringing cables between spires and ledges and masts and walking across without a net. On August 7 1974, he achieved his masterpiece: walking across the towers of the World Trade Centre in downtown New York as a stunned crowd gathered below. He and his crew had had to creep up both structures in twin teams, and then attach the wire by literally firing across the initial guiding rope from one tower to the other with a bow and arrow. It was the epat to end all epats : a sensational piece of victimless criminal daring which required enormous cunning and discipline, not merely in the extraordinary act itself - Petit impishly danced back and forth across the wire over and over again while fuming cops raged near the ledge - but in the preparation and the skulduggery involved smuggling in the gear and disguised personnel, as if for a bank job.

What Petit brought off was a remarkable, even religious gesture of devotion, both to the building and to New York itself; this was, in fact, a unique act of homage no other artist could have managed, and New Yorkers instantly appreciated it. Graham Greene once playfully endorsed the Great Train Robbers' crime, but his praise for these violent men was misjudged; I wonder if he missed a trick in not writing about Petit, instead?

James Marsh's documentary about this sublime piece of audacity does full justice to Petit's vision, using interviews with the man himself and his crew, and using photos from the time, and dramatised reconstructions - there is evidently no home-movie record and no television footage, as this was before the age of rolling coverage and rapid-response news 'copters.

At this point, it has to be said that there is an elephant-in-the-living-room aspect to discussing Petit's great coup: namely, its similarity in some ways to a very much more malign spectacular brought off at the same location 27 years later. But with shrewdness and elegance, a defiant insistence on the subject's purity, Marsh tacitly allows us to realise the various parallels but says not a word about them. So neither will I.

Petit was an artist and a genius: the WTC exploit surely entitles him to both those descriptions. He describes how he conceived a fascination with the World Trade Centre towers even before they were built, reading about the plans in a magazine in a dentist's waiting-room as a boy. He claims that there was something in the buildings that cried out for a tightrope walker's wire to be strung between them. They were built to be used as he wished to use them: a successful high-wire walk would fulfil not merely his own destiny, but that of the two towers themselves. They were like those geographical areas in his In Search of Lost Time that Proust said were predestined to be battlefields because of accidents of geological formation: rivers, rises, gullies, which both hinder and inspire a general or tactician: "You don't make an artist's studio out of any old room; so you don't make a battlefield out of any old piece of ground."

His planning was extraordinarily detailed, involving many recce trips and dummy runs and even an entire fake magazine-journalist expedition, in which, posing as a reporter, he interviewed construction workers at the top of the yet-unfinished structure while his photographer took photos of these men, and also, covertly, photos of the ledges and the structures they would need for the rigging and the harness. Heartbreakingly, I notice they did get some cine-film of this cheeky exploit, but somehow failed to get any of the main event.

What of the aftermath? Petit relied heavily on various faintly dodgy and unreliable local American guys to get him into the building, but the actual technicians of the walk were his tried-and-trusted equipe : Jean-Francois Heckel and Jean-Louis Blondeau. There was also his devoted, gentle girlfriend Annie Allix. Their testimony is somehow unbearably moving - they are awestruck and tearful even now, though Petit is just cordial and ebullient. But what is even more painful is the fact that though big-hearted New Yorkers fell in love with the crazy Frenchman Petit, there was no celebrity status accorded to his humble helpers, who wound up being treated slightingly. Petit even betrayed Annie by having a fling with a beautiful American fan. Could it be that though Petit did not fall, there were others who did?

What Marsh shows us is Petit's childlike innocence and almost transcendental faith: faith in himself, faith in his leadership abilities, faith that the escapade would be a success, and faith that he would not fall. His sheer hypnotic self-belief meant that I found it quite impossible to imagine him losing his balance and plunging to his death: he defies gravity. In our world of elf'n'safety, a world where success and fame means working within very well-understood corporate structures, Petit is a rare, exotic beast, and a wonderful one.